__cxa_demangle terminates the program on failure to allocate

[ldionne]

As explained in https://reviews.llvm.org/D155598, we currently terminate the program after failing to (re)allocate inside the Itanium demangler. This happens when we fail to grow the OutputBuffer inside ItaniumDemangle.h.

Instead, we should properly handle failure-to-allocate inside ItaniumDemangle.h and propagate something like an error code or a std::expected to the caller of all functions in the demangler that can allocate. Then, we could make sure that __cxa_demangle returns nullptr when it fails to demangle the input due to any error, as it is supposed to.